The
Fascinating World of Salt and Ice..An
easy-to-understand explanation with pictures of why putting salt on
ice melts it and makes ice/water mixtures colder.

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Combining
ice and salt produces a remarkable yet easily understood reaction
that is important both to making ice cream at home and anywhere salt
is used on roads to melt ice. To understand this phenomena we first
need to take a close look at a plain ice-and-water mixture.

In the picture
above the densely packed blue spheres on the left represent water
molecules solidified into ice, the looser spheres on the right are
water molecules. Ice consists of water molecules locked in a
crystalline structure. Liquid water consists of free water molecules
randomly moving around in all directions.
(Note: in reality ice is less dense than water,
which is why it floats. As water freezes the growing crystaline
structure forces water molicules further apart than they are in
liquid water. I've drawn the ice as close-packed molecules to
simplify the image.)

Assuming there
are no outside sources of heat or cold, the ice and water will be in
equilibrium. However, this doesn't mean that everything is static.
Every once in a while a free-swimming water molecule will collide
with an ice molecule and in so doing give it sufficient energy to
break away from the ice block. In other words that molecule just
melted. Because the ice molecules are stuck together it takes energy
to break a molecule free. In this case the energy came from the free-swimming
water molecule that collided with the ice. It had a certain amount
of energy stored in the form of kinetic energy, the energy it
possessed because it was moving. (Just like a moving car has energy.)
After the collision, the original free-swimming molecule will be
moving slower and therefore have less energy. The difference between
the amount of energy it had before and after the collision is same as
the amount of energy it took to break the frozen ice molecule free.
Imagine shooting a bullet through a pyramid of soda cans. As it
knocks one of the cans off the stack it slows down a little. The can
absorbed some of the bullet's energy to get dislodged from the rest
of the cans.

In water, the
temperature of the water is a measure of how fast the average water
molecule is moving. The hotter the water, the faster the molecules
move. When the water molecule above collided with the ice and as a
result slowed down, it can now be thought of as being cooler. If this
happens to enough water molecules the water as a whole will measure cooler.

Just as
free-swimming water molecules can break loose ice molecules, so they
themselves can collide with the ice in a way that causes them to
stick to it, or freeze. In so doing they release energy to the
water/ice mixture from their loss of kinetic energy and the energy
given up as they create a molecular bond with the ice. This energy
goes into the remaining water, making the molecules travel faster and
in effect: heating it up.

In a stable
mixture of water and ice the rate of ice molecules being broken free,
and taking energy from the mixture, and water molecules freezing to
the ice, and giving up energy to the mixture, is the same so that
over time both the amount of water and ice and the temperature (a
measure of the kinetic energy in the system) is constant.

But, let's say
you want to melt the ice on your driveway. You sprinkle a little salt
on it and like magic the ice starts to melt. Why?

Consider the
following image:

Here we have
the same water/ice mixture we had in the first picture, except this
time salt (black spheres) has dissolved into the water and displaced
some of the water molecules. Although the water now contains some
salt molecules, the same number of collisions occur on the ice as
before, maintaining the same rate of ice molecules being freed to
join the free-swimming water molecules. BUT, because the salt
molecules have replaced some of the water molecules in the water not
as many water molecules collide with the ice and therefore there are
fewer of them that can freeze to it. (The crystalline structure of
the ice prohibit salt molecules striking the ice from becoming
incorporated into it. The nature of the molecular bonds repels the
salt.) Net result: there are more ice molecules melting than there
are water molecules freezing. Because the salt isn't used up in this
process the melting continues as long as the water flowing from the
ice doesn't wash the salt away. This is why only a little salt is
needed to melt a lot of ice.

A
Simple Experiment
That Shows This Happening:

Take
an ice cube and sprinkle a pinch of salt on one-half of it and leave
it in the refrigerator for ten minutes. In the image below a little
salt was sprinkled on the left half of the cube:

A
little while later the side with salt on it has melted much more
than the unsalted side.

A
close look shows that each salt grain creates its own tunnel down
into the ice.

One curiosity
about this process is that contrary to our everyday experience, the
melting ice is actually colder than it was before the salt was added.
There reason this is so is the same reason salt is added to home ice
cream makers: to make the water/ice mixture cold enough to freeze the
ice cream, which freezes below the temperature of normal ice.

To understand
why the water/ ice mixture gets colder when salt is added we need to
remember that as ice molecules melt they absorb energy from the
mixture by causing the water molecules to slow down. Equally, when
water freezes to the ice it gives up its energy and in so doing
causes the surrounding molecules to speed up a little, or in other
words get hotter. In a pure water/ice mixture the rate of both
processes is equal so the mixture doesn't heat up or get colder. When
salt is added the rate of melting stays the same and therefore so
does the rate of energy being absorbed from the mixture. But, the
rate of water molecules freezing to the ice is reduced and therefore
so is the amount of energy being released by this process. Net
result: more melting than freezing means that more energy is being
absorbed by the mixture than is being released to it. Net result: the
mixture gets colder.

Think of it
like this: we all know it takes energy to melt ice. Normally we do
this by adding heat. But, in the water/ice/salt situation the only
source of heat (or energy) to melt the ice is the kinetic energy
stored in the water and ice mixture itself. If some of that energy is
used up breaking the molecular bonds between ice molecules then the
total genetic energy of the system, as measured by its temperature,
must decrease.

IMPORTANT
NOTE:

The
subject of ice is much more complex that the simplified explanation
on this page suggests. For more detailed treatments try: http://www.chem1.com/acad/sci/aboutwater.html
and http://www.lsbu.ac.uk/water/index2.html.

(Click
here to browse 70 topics on my main
site ranging from magnetic rocket engines and more kaleidoscopes
pages to the strange world of lucid dreaming.)